1. Field of the Invention
The present invention relates to a printed wiring board device on which solder junctions of individual parts are made by using reflow equipment, and to a method of manufacture of the printed wiring board device.
2. Description of the Related Art
In recent years, there is the trend of promoting the activities that do not use the toxic substances having undesired influences on the environment, with an increase of the concern about environmental problems. In the field of manufacture of printed wiring boards, such as electronic devices, the activities to eliminate the toxic lead substances that are derived from the conventional solder (e.g., 37 Pbxe2x80x94Sn eutectic alloy solder) are also promoted. The use of the unleaded solder that does not contain such lead substances is proposed for the field of manufacture of printed wiring boards, instead of the leaded solder.
From the standpoint of chip packaging techniques, if the leaded solder (the eutectic alloy solder) is replaced by the unleaded solder, the melting point of the unleaded solder is raised from 183 deg. C. that is the melting point of the eutectic alloy solder. For example, the melting point of the Snxe2x80x94Zn unleaded solder goes up to 200 deg. C., and the melting point of the Snxe2x80x94Agxe2x80x94Cu unleaded solder goes up to 217-221 deg. C. In connection with this, the mounting temperature that ensures good solder junctions in the case of the unleaded solder is also raised by about 10-20 deg. C. from 205 deg. C. that is the mounting temperature in the case of the leaded solder.
By the rise of the mounting temperature conditions for ensuring good solder junctions, the reliability of individual parts of the printed wiring board is significantly affected. Generally, the heat-resistant temperature of the printed wiring board parts varies depending on the composition material and size of the individual parts. Even if the leaded solder is replaced by the unleaded solder, the heat-resistant temperature remains unchanged. Hence, it is necessary to take measures to reduce the time to maintain the high temperature conditions as small as possible for the printed wiring board parts wherein the permissible temperature conditions are severe.
Especially when performing reflow soldering of a printed wiring board including isolated parts that are arranged at positions isolated from other parts (the non-isolated or thinly populated parts) on the wiring board, there is a problem in that, if the setting of the reflow equipment is adjusted to the temperature conditions that ensure good solder junctions for the non-isolated parts, the reflow heating temperature, created by the reflow equipment, exceeds a heat-resistant temperature of the isolated parts.
If the reflow heating temperature exceeds the heat-resistant temperature of the isolated parts, peeling or swelling of the resin layer at terminal connections in the isolated parts or the like will occur and a degradation of the performance over an extended period of time will be caused. For this reason, the conventional circuit parts should not be used without change. It is necessary to change the conventional circuit parts to special circuit parts using a composition material having a high thermal resistance or to improve the manufacturing processes by taking preventive measures against the peeling or swelling of the resin layer of the isolated parts.
Japanese Laid-Open Patent Application Nos. 2001-85827, 8-64953 and 8-236913 disclose improvements of pads and circuit patterns for a printed wiring board that is configured to suppress the temperature variations of the lead portion of each individual part on the printed wiring board to obtain the uniform temperature, and to suppress irregularities of solder junctions at the time of soldering.
Moreover, Japanese Published Utility Model Application No. 7-37337 discloses an electronic circuit chip in which dummy parts are provided. However, in this circuit chip, the dummy parts are provided in order to prevent the engagement between adjacent circuit chips when arranging the circuit chips in row into a cylindrical case and mounting the circuit chips on a printed circuit board by using an automatic mounting machine.
It should be noted that the conventional methods of the above-mentioned documents system do not take into consideration about preventive measures against such problem that the reflow heating temperature, created by the reflow equipment, exceeds the heat-resistant temperature of the isolated parts when the temperature conditions of the reflow equipment are made severer.
A conceivable method for improvement of the manufacturing processes is that a special jig is provided in order to prevent the heat concentration on specific circuit parts. However, it is difficult to carry out the temperature control with the use of the special jig while maintaining the accuracy of the temperature control reasonably high, and such improvement is disadvantageous with respect to the manufacture cost.
An object of the present invention is to provide an improved printed wiring board device in which the above-described problems are eliminated.
Another object of the present invention is to provide a printed wiring board device that prevents the temperature rise of the circuit parts during the reflow soldering from exceeding the heat-resistant temperature thereof, and maintains the circuit parts in proper temperature conditions, thereby ensuring that good solder junctions are made on the wiring board.
Another object of the present invention is to provide a method of manufacturing a printed wiring board device that prevents the temperature rise of the circuit parts during the reflow soldering from exceeding the heat-resistant temperature thereof, and maintains the circuit parts in proper temperature conditions, thereby ensuring that good solder junctions are made on the wiring board.
Another object of the present invention is to provide an information processing apparatus in which a printed wiring board device is provided, the printed wiring board device preventing the temperature rise of the circuit parts during the reflow soldering from exceeding the heat-resistant temperature thereof, and maintaining the circuit parts in proper temperature conditions, thereby ensuring that good solder junctions are made on the wiring board.
The above-mentioned objects of the present invention are achieved by a printed wiring board device comprising: circuit parts which are mounted on a wiring board and constitute parts of a circuit in the printed wiring board device; pads which are provided on the wiring board in the circumference of the circuit parts and electrically isolated from the circuit; and heat-absorbing dummy parts which are mounted on the pads and absorb heat from the circuit parts through the pads during reflow soldering.
The above-mentioned objects of the present invention are achieved by a printed wiring board device comprising: circuit parts which are mounted on a wiring board at positions isolated from other parts on the wiring board, the circuit parts constituting parts of a circuit in the printed wiring board device; and heat-absorbing dummy parts which are mounted on the wiring board by using an adhesive agent that is thermally conductive, the dummy parts absorbing heat from the circuit parts through the adhesive agent during reflow soldering.
The above-mentioned objects of the present invention are achieved by a method of manufacturing a printed wiring board device having circuit parts mounted on a wiring board and constituting parts of a circuit in the printed wiring board device, the method comprising steps of: providing pads on the wiring board in the circumference of the circuit parts such that the pads are electrically isolated from the circuit; applying a solder to the wiring board in accordance with a wiring pattern of the wiring board; mounting heat-absorbing dummy parts on the pads; and performing a reflow soldering of the printed wiring board device to produce solder junctions on the wiring board by using a reflow equipment, wherein the dummy parts absorbs heat from the circuit parts through the pads during the reflow soldering, thereby preventing an excessive temperature rise of the circuit parts.
The above-mentioned objects of the present invention are achieved by an information processing apparatus in which a printed wiring board device is provided, the printed wiring board device comprising: circuit parts which are mounted on a wiring board and constitute parts of a circuit in the printed wiring board device; pads which are provided on the wiring board in the circumference of the circuit parts and electrically isolated from the circuit; and heat-absorbing dummy parts which are mounted on the pads and absorb heat from the circuit parts through the pads during reflow soldering.
In the printed wiring board device of the present invention, the circuit parts are mounted on the wiring board and constitute parts of the circuit on the wiring board. The pads are provided on the wiring board in the circumference of the circuit parts and electrically isolated from the circuit. The heat-absorbing dummy parts are mounted on the pads and absorb heat from the circuit parts through the pads during the reflow soldering. Even if the circuit parts having a low thermal resistance are needed to be isolated from other parts on the wiring board, it is possible to prevent the temperature rise of the circuit parts during the reflow soldering from exceeding the heat-resistant temperature thereof. The printed wiring board device of the present invention is effective in maintaining the circuit parts in proper temperature conditions, and it can ensure that good solder junctions are made on the wiring board. The mounting of the dummy parts on the wiring board can be easily carried out, and it contributes to reduction of the manufacturing cost while it prevents the temperature rise of the circuit parts during the reflow soldering.